The field of the invention is the preparation of carboxylic acids from hydrocarbons. The present invention is particularly concerned with a process for the preparation of acetic acid by a catalytic gas-phase oxidation of butenes.
The state of the art may be ascertained by reference to the Kirk-Othmer Encyclopedia of Chemical Technology, 2nd Ed., Vol. 8 (1966), under the section "Ethanoic Acid," pages 386-404, particularly page 396, page 397, and FIG. 5 on page 396. FIG. 5 of Kirk-Othmer discloses the process for the liquid phase oxidation of butenes to acetic acid.
The state of the prior art processes for the production of acetic acid by gas-phase oxidation of butenes in the presence of vanadate catalysts may be ascertained by reference to U.S. Pat. Nos. 3,431,297; 3,439,029; and 3,459,797, the disclosures of which are incorporated herein.
The state of the art may be further ascertained by reference to German Published Applications Nos. 1,069,150 (corresponding to British Patent No. 887,667) and 1,280,865; German Pat. Nos. 1,269,119 (corresponding to U.S. Pat. No. 3,439,029); 1,271,104 (corresponding to U.S. Pat. No. 3,459,797); and 1,279,011 (corresponding to U.S. Pat. No. 3,431,297): German unexamined published applications Nos. 1,903,190; 1,921,503; and 2,016,681, and Belgian Pat. No. 723,652.
Processes for the production of acetic acid are known from German Pat. Nos. 1,269,119 (corresponding to U.S. Pat. No. 3,439,029); 1,271,104 (corresponding to U.S. Pat. No. 3,459,797); and 1,279,011 (corresponding to U.S. Pat. No. 3,431,297), wherein butenes are reacted in the gaseous phase in the presence of a vanadate catalyst with oxygen or with oxygen-containing gases at an elevated temperature in the presence of steam. The vanadate catalysts are mixed oxides of vanadium with tin, antimony, titanium, or aluminum or vanadium metal and oxide mixtures of these metals.
In the oxidation of hydrocarbons, the final products which are thermodynamically stable are always carbon dioxide and water. The desired products, such as for example, oxides, anhydrides, or acids, are intermediate products which have only a relative stability toward a further oxidation under the reaction conditions (see inter alia, German Published Application No. 1,280,865, column 3, line 49, to column 4, line 19). For this reason, in oxidation processes of this type such as, for example, in the oxidation of ethylene to ethylene oxide or the oxidation of benzene to maleic anhydride (see German Published Application No. 1,069,150), when a cycle gas operation is used, a portion of the efflux stream leaving the reactor called cycle gas is recycled into the reactor after substantially a complete separation of the desired oxidation products.
In the production of acetic acid according to the afore-mentioned German Pat. Nos. 1,269,119 (see column 3, lines 24-28), 1,271,104 (see column 2, line 46, to column 3, line 2, as well as Example 2), and 1,279,011 (see column 3, lines 3--7), when cycle gas is used, the gaseous stream exiting from the reactor is freed of the acetic acid produced first, and then the cycle gas is branched off. In German Unexamined Published Application No. 1,921,503, which is also concerned with a process for the preparation of acetic acid by catalytic gas-phase oxidation of butenes, it is, however, disclosed that according to the state of the art a cycle gas operating method is uneconomical and therefore is no longer contemplated by those skilled in the art.
According to the prior art processes incorporated herein, the catalytic gas-phase oxidation of butene to acetic acid is conducted in the presence of steam, since it has been discovered (see German Pat. Nos. 1,269,119; 1,271,104; and 1,279,011) that the selectivity with respect to the formation of acetic acid is enhanced by steam in the reaction mixture.
When the cycle gas mode of operation is effected, according to the prior art, by separating the acetic acid from the reaction gas by condensation, water contained in the reaction gas is substantially removed. As a result, steam must be added to the reaction gas at the reactor inlet in order to maintain the steam concentration required for a sufficient selectivity for acetic acid formation in the reaction chamber. This means that, together with the acetic acid, the previously added water is obtained in the condensate along with the water formed during the side reactions resulting in CO and CO.sub.2. Consequently, in this prior art mode of operation, it is possible to obtain only a moderately concentrated acetic acid with a maximum concentration of 20 percent by weight. The dehydration of the crude acid in accordance with known separating methods, such as, for example, azeotropic rectification with benzene or ethyl n-butyl ether, or liquid-liquid extraction with diisopropyl ether is, however, more expensive, the higher the water content of the crude acid. Since the above-described prior art process leads only to dilute acetic acid, the usefulness of this process on a commercial scale depends greatly on the cost of the dehydration of the acetic acid and thus on the state of the art with respect to dehydration processes.
An additional disadvantage in the above-mentioned mode of operation with cycle gas resides in that the entire gas exiting from the reactor must be cooled in a heat exchanger, to condense it, in order to separate the acetic acid substantially from the reaction gas. As a result of this required cooling to about 20.degree. - 30.degree.C, a large amount of the enthalpy of the gas cannot be exploited. Moreover, the use of cooling media requires considerable expense. Furthermore, a large heat exchange surface is required for cooling the entire gas, which is a considerable cost factor, especially since, in the case of acetic acid production, expensive acid-proof materials must be employed.
Attempts have been made to design the process in such a way that a more concentrated acetic acid can be obtained. In Belgian Pat. No. 723,652, a process is described wherein, by connecting several reactors in series and by feeding the butene upstream of each reactor, an acid of a higher concentration is obtained. Thus, a 31 percent by weight acid is produced in the oxidation of butene by connecting four reactors in series, but wherein after the third reactor a portion of the reaction product is condensed and withdrawn. If a higher acetic acid concentration is desired, a larger number of reactors must be connected one behind the other, which is disadvantageous for the design of a large commercial plant.